This application claims the benefit of Japanese Application No. 2001-105528 filed Apr. 4, 2001.
The present invention relates to a method of correcting a resonance frequency variation and an MRI (Magnetic Resonance Imaging) apparatus and, more specifically, to a method of correcting a resonance frequency variation, which is capable of handling all frequency drifts caused by fluctuations in magnetic field including a frequency drift whose time change is slow, a frequency drift in a slice direction and a frequency drift whose time change is fast.
As a first prior art, Japanese Laid-open Patent Publication No. 1-141656 discloses a technology for correcting a frequency drift (resonance frequency variation) caused by fluctuations in magnetic field by collecting correction data as well at the time of collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding quantity a plurality of times and adjusting the amount of a current running through a magnetostatic coil based on the collected correction data.
As a second prior art, Japanese Patent No. 2528864 disclose a technology for making phase correction operation on imaging data according to magnetostatic intensity by measuring and storing the magnetostatic intensities of a plurality of points.
In the above first prior art, since correction based on correction data collected at the time of a sequence of imaging pulses of a certain time cannot be in time for the imaging data of this time and is effective for imaging data of the next time, a frequency drift whose time change is fast cannot be handled though a frequency drift whose time change is slow has no problem.
In the above second prior art, since imaging data collected at a certain time can be corrected based on correction data collected at the time of a sequence of imaging pulses of that time, a frequency drift whose time change is fast can be handled. However, since the processing is made after the collection of data, correction is effective for a sliced surface alone and a frequency drift in a slice direction cannot be handled.
It is therefore an object of the present invention to provide a method of correcting a resonance frequency variation, which is capable of handling all frequency drifts including a frequency drift whose time change is slow, a frequency drift in a slice direction and a frequency drift whose time change is fast as well as an MRI apparatus.
According to a first aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well each time when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation when the amount of the resonance frequency variation is smaller than a threshold value, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time without correcting the frequency variation when the amount of the resonance frequency variation is not smaller than the threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
When the amount of the resonance frequency variation is smaller than the threshold value, as the resonance frequency variation is not a frequency drift whose time change is fast, it will suffice without correcting the imaging data of this time if the correction of the imaging data of the next time is effective. Therefore, only the correction of a frequency variation has to be carried out and the amount of a resonance frequency variation or resonance frequency does not need to be stored. On the other hand, when the amount of the resonance frequency variation is not smaller than the threshold value, as the resonance frequency variation is a frequency drift whose time change is fast, correction must be made on the imaging data of this time as well. Therefore, the amount of the resonance frequency variation or the resonance frequency is stored and correction operation may be made based thereon.
The method of correcting a resonance frequency variation according to the first aspect of the present invention is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. Meanwhile, since correction operation is made on a frequency drift whose time change is fast later, the imaging data of this time can be corrected. When correction operation is made later, frequency variation correction is not made to simplify control.
According to a second aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well each time when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time when the amount of the resonance frequency variation is not smaller than a threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above second aspect of the present invention is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. Meanwhile, since correction operation is made on a frequency drift whose time change is fast later, the imaging data of this time can be corrected. Since frequency variation correction is made even when correction operation is made later, a frequency drift in a slice direction can always be handled.
According to a third aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well once out of a plurality of times when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation when the amount of the resonance frequency variation is smaller than a threshold value, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times without correcting the frequency variation when the amount of the resonance frequency variation is not smaller than the threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above third embodiment is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. Meanwhile, since correction operation is made on a frequency drift whose time change is fast later, imaging data of this time can be corrected. When correction operation is made later, frequency variation correction is not made to simplify control. Since the amount of a resonance frequency variation is measured once every a plurality of sequences of imaging pulses, the total scanning time can be shortened.
According to a fourth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well once out of a plurality of times when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times when the amount of the resonance frequency variation is not smaller than a threshold value, and making correction operation on the imaging data when an amount of a resonance frequency variation or resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above fourth aspect of the present invention is based on the above principle and makes frequency variation correction on a frequency drift whose time change is slow, thereby making it possible to handle a frequency drift in a slice direction as well. On the other hand, since correction operation is made on a frequency drift whose time change is fast later, the imaging data of this time can be corrected. Meanwhile, since frequency variation correction is carried out even when correction operation is made later, a frequency drift in a slice direction can always be handled. Since the amount of a resonance frequency variation is measured only once every a plurality of sequences of imaging pulses, the total scanning time can be shortened.
According to a fifth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well each time when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time, and making correction operation on the imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above fifth aspect of the present invention is based on the above principle and carries out frequency variation correction and also makes correction operation later, thereby making it possible to handle both a frequency drift whose time change is slow and a frequency drift whose time change is fast.
According to a sixth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, comprising the steps of, measuring an amount of a resonance frequency variation as well once out of a plurality of times when imaging data for filling a k space is collected by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times, correcting the frequency variation, storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times, and making correction operation on the imaging data after the collection of the imaging data for filling the k space.
The method of correcting a resonance frequency variation according to the above sixth aspect of the present invention is based on the above principle and carries out frequency variation correction and also makes correction operation later, thereby making it possible to handle both a frequency drift whose time change is slow and a frequency drift whose time change is fast.
According to a seventh aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein the resonance frequency is measured and a difference between the measured resonance frequency and a fixed resonance frequency is taken as the amount of the resonance frequency variation in the above method of correcting a resonance frequency variation.
Since a difference between the measured resonance frequency and the fixed resonance frequency is taken as the amount of the resonance frequency variation in the above method of correcting a resonance frequency variation according to the above seventh aspect of the present invention, the resonance frequency is easily obtained from the amount of the resonance frequency variation.
According to an eighth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein the resonance frequency is measured and a difference between the measured resonance frequency and the previously measured resonance frequency is taken as the amount of the resonance frequency variation in the above method of correcting a resonance frequency variation.
Since a difference between the measured resonance frequency and the previously measured resonance frequency is taken as the amount of the resonance frequency variation in the method of correcting a resonance frequency variation according to the eight aspect of the present invention, the size of the variation can be easily grasped.
According to a ninth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein the threshold value is a fixed value in the above method of correcting a resonance frequency variation.
Since the threshold value is a fixed value in the method of correcting a resonance frequency variation according to the ninth aspect of the present invention, the processing becomes easy.
According to a tenth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein the threshold value is altered according to a change in the amount of the resonance frequency variation in the above method of correcting a resonance frequency variation.
Since the threshold value is altered according to a change in the amount of the resonance frequency variation in the method of correcting a resonance frequency variation according to the tenth aspect of the present invention, the threshold value can be dynamically optimized.
According to an eleventh aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein the amount of a current running through a main magnetic field coil is adjusted to correct the frequency variation in the above method of correcting a resonance frequency variation.
Since the amount of a current running through the main magnetic coil is adjusted to correct a frequency drift in the method of correcting a resonance frequency variation according to the eleventh aspect of the present invention, the image quality can be improved.
According to a twelfth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein a transmitting frequency is adjusted to correct the frequency variation in the above method of correcting a resonance frequency variation.
Since a transmitting frequency is adjusted to correct a frequency drift in the method of correcting a resonance frequency variation according to the eleventh aspect of the present invention, the image quality can be improved.
According to a thirteenth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein the transmitting frequency and a receiving frequency are adjusted to correct the frequency variation in the above method of correcting a resonance frequency variation.
Since the transmitting frequency and the receiving frequency are adjusted to correct a magnetic field drift in the method of correcting a resonance frequency variation according to the thirteenth aspect of the present invention, the image quality can be improved.
According to a fourteenth aspect of the present invention, there is provided a method of correcting a resonance frequency variation, wherein a transmitting phase or a receiving phase is adjusted to correct the frequency variation in the above method of correcting a resonance frequency variation.
Since a transmitting phase or a receiving phase is adjusted to correct a magnetic field drift in the method of correcting a resonance frequency variation according to the fourteenth aspect of the present invention, the image quality can be improved.
According to a fifteenth aspect of the present invention, there is provided an MRI APPARATUS comprising, RF pulse transmitting means, gradient pulse application means, NMR signal receiving means, imaging data collection control means for collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times by controlling the above means, frequency variation measurement control means for measuring an amount of a resonance frequency variation as well each time when the imaging data is collected, frequency variation correction means for correcting a frequency variation when the amount of the resonance frequency variation is smaller than a threshold value, frequency variation storage means for storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time when the amount of the resonance frequency variation is not smaller than the threshold value, correction operation means for making correction operation on the imaging data when the amount of the resonance frequency variation or the resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space, and reconstruction operation means for reconstructing an image from imaging data which does not require correction operation and imaging data after correction operation.
With the MRI APPARATUS according to the above fifteenth aspect of the present invention, the method of correcting a resonance frequency variation according to the first aspect of the present intention can be advantageously carried out.
According to a sixteenth aspect of the present invention, there is provided an MRI APPARATUS comprising, RF pulse transmitting means, gradient pulse application means, NMR signal receiving means, imaging data collection control means for collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times by controlling the above means, frequency variation measurement control means for measuring an amount of a resonance frequency variation as well each time when the imaging data is collected, frequency variation correction means for correcting a frequency variation, frequency variation storage means for storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time when the amount of the resonance frequency variation is not smaller than a threshold value, correction operation means for making correction operation on the imaging data when the amount of the resonance frequency variation or the resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space, and reconstruction operation means for reconstructing an image from imaging data which does not require correction operation and imaging data after correction operation.
With the MRI APPARATUS according to the above sixteenth aspect of the present invention, the method of correcting a resonance frequency variation according to the above second aspect of the present invention can be advantageously carried out.
According to a seventeenth aspect of the present invention, there is provided an MRI APPARATUS comprising, RF pulse transmitting means, gradient pulse application means, NMR signal receiving means, imaging data collection control means for collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times by controlling the above means, frequency variation measurement control means for measuring an amount of a resonance frequency variation as well once out of a plurality of times when the imaging data is collected, frequency variation correction means for correcting a frequency variation when the amount of the resonance frequency variation is smaller than a threshold value, frequency variation storage means for storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times when the amount of the resonance frequency variation is not smaller than the threshold value, correction operation means for making correction operation on the imaging data when the amount of the resonance frequency variation or the resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space, and reconstruction operation means for reconstructing an image from imaging data which does not require correction operation and imaging data after correction operation.
With the MRI APPARATUS according to the above seventeenth aspect of the present invention, the method of correcting a resonance frequency variation according to the above third aspect of the present invention can be advantageously carried out.
According to an eighteenth aspect of the present invention, there is provided an MRI APPARATUS comprising, RF pulse transmitting means, gradient pulse application means, NMR signal receiving means, imaging data collection control means for collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times by controlling the above means, frequency variation measurement control means for measuring an amount of a resonance frequency variation as well once out of a plurality of times when the imaging data is collected, frequency variation correction means for correcting a frequency variation, frequency variation storage means for storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times when the amount of the resonance frequency variation is not smaller than a threshold value, correction operation means for making correction operation on the imaging data when the amount of the resonance frequency variation or the resonance frequency is stored in correspondence to a certain phase encoding amount of imaging data after the collection of the imaging data for filling the k space, and reconstruction operation means for reconstructing an image from imaging data which does not require correction operation and imaging data after correction operation.
With the MRI APPARATUS according to the above eighteenth aspect of the present invention, the method of correcting a resonance frequency variation according to the above fourth aspect of the present invention ca be advantageously carried out.
According to a nineteenth aspect of the present invention, there is provided an MRI APPARATUS comprising, RF pulse transmitting means, gradient pulse application means, NMR signal receiving means, imaging data collection control means for collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times by controlling the above means, frequency variation measurement control means for measuring an amount of a resonance frequency variation as well each time when the imaging data is collected, frequency variation correction means for correcting a frequency variation, frequency variation storage means for storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of this time, correction operation means for making correction operation on the imaging data after the collection of the imaging data for filling the k space, and reconstruction operation means for reconstructing an image from imaging data after correction operation.
With the MRI APPARATUS according to the above nineteenth aspect of the present invention, the method of correcting a resonance frequency variation according to the above fifth aspect of the present invention can be advantageously carried out.
According to a twentieth aspect of the present invention, there is provided an MRI APPARATUS comprising, RF pulse transmitting means, gradient pulse application means, NMR signal receiving means, imaging data collection control means for collecting imaging data for filling a k space by repeating a sequence of imaging pulses which differ in phase encoding amount a plurality of times by controlling the above means, frequency variation measurement control means for measuring an amount of a resonance frequency variation as well once out of a plurality of times when the imaging data is collected, frequency variation correction means for correcting a frequency variation, frequency variation storage means for storing the amount of the resonance frequency variation or the resonance frequency in correspondence to the imaging data of the plurality of times, correction operation means for making correction operation on the imaging data after the collection of the imaging data for filling the k space, and reconstruction operation means for reconstructing an image from imaging data after correction operation.
With the MRI APPARATUS according to the above twentieth aspect of the present invention, the method of correcting a resonance frequency variation according to the above sixth aspect of the present invention can be advantageously carried out.
According to a twenty-first aspect of the present invention, there is provided an MRI apparatus, wherein the above frequency variation measurement control means measures a resonance frequency and a difference between the measured resonance frequency and a fixed resonance frequency is taken as the amount of the resonance frequency variation in the above MRI apparatus.
With the MRI apparatus according to the above twenty-first aspect of the present invention, the method of correcting a resonance frequency variation according to the above seventh aspect of the present invention can be advantageously carried out.
According to a twenty-second aspect of the present invention, there is provided an MRI APPARATUS, wherein the above frequency variation measurement control means measures a resonance frequency and a difference between the measured resonance frequency and the previously measured resonance frequency is taken as the amount of the resonance frequency variation in the above MRI apparatus.
With the MRI apparatus according to the above twenty-second aspect of the present invention, the method of correcting a resonance frequency variation according to the above eighth aspect of the present invention can be advantageously carried out.
According to a twenty-third aspect of the present invention, there is provided an MRI apparatus, wherein the threshold value is a fixed value in the above MRI apparatus.
With the MRI apparatus according to the above twenty-third aspect of the present invention, the method of correcting a resonance frequency variation according to the above ninth aspect of the present invention can be advantageously carried out.
According to a twenty-fourth aspect of the present invention, there is provided an MRI apparatus, wherein threshold value altering means for altering the threshold value according to a change in the amount of a resonance frequency variation is provided in the above MRI apparatus.
With the MRI apparatus according to the above twenty-fourth aspect of the present invention, the method of correcting a resonance frequency variation according to the above tenth aspect of the present invention can be advantageously carried out.
According to a twenty-fifth aspect of the present invention, there is provided an MRI apparatus, wherein the above frequency variation correction means adjusts an amount of a current running through a main magnetic field coil in the above MRI apparatus.
With the MRI apparatus according to the above twenty-fifth aspect of the present invention, the method of correcting a resonance frequency variation according to the above eleventh aspect of the present invention can be advantageously carried out.
According to a twenty-sixth aspect of the present invention, there is provided an MRI apparatus, wherein the above frequency variation correction means adjusts a transmitting frequency in the above MRI apparatus.
With the MRI apparatus according to the above twenty-sixth aspect of the present invention, the method of correcting a resonance frequency variation according to the above twelfth aspect of the present invention can be advantageously carried out.
According to a twenty-seventh aspect of the present invention, there is provided an MRI apparatus, wherein the above frequency variation correction means adjusts the transmitting frequency and a receiving frequency in the above MRI apparatus.
With the MRI apparatus according to the above twenty-seventh aspect of the present invention, the method of correcting a resonance frequency variation according to the above thirteenth aspect of the present invention can be advantageously carried out.
According to a twenty-eighth aspect of the present invention, there is provided an MRI apparatus, wherein the above frequency variation correction means adjusts a transmitting phase or a receiving phase in the above MRI apparatus.
With the MRI apparatus according to the above twenty-eighth aspect of the present invention, the method of correcting a resonance frequency variation according to the above fourteenth aspect of the present invention can be advantageously carried out.
Therefore, according to the method of correcting a resonance frequency variation and MRI apparatus of the present invention, all frequency drifts including a frequency drift whose time change is slow, a frequency drift in a slice direction and a frequency drift whose time change is fast can be handled, thereby making it possible to improve image quality.